Massive flood barriers may not be the most cost-effective way to control severe flooding from future Sandy-like storms in New York City, at least in the next two decades, according to a new analysis from Dutch and American researchers.

However, the city needs to be planning for those types of huge barriers more as part of a longer-term plan, and as preparation for the possibility that climate change and sea-level rise may be worse than expected, warns the analysis, published last week in Science. In the near term before 2040, the most cost-effective way to protect New York likely is a more modest combination of building improvements—such as raising new structures and home foundations—and smaller barrier methods to keep out water, such as small levees, the policy study finds.

"The city is on a good path ... but it needs to be studying the barriers," said Jeroen Aerts, a professor of risk management at the Institute for Environmental Studies in Amsterdam and lead author of the analysis, which also included researchers from the Massachusetts Institute of Technology, Princeton University and the University of Pennsylvania. "Risks increase very rapidly due to climate change."

The analysis of future flood options for New York City does not capture all the psychological factors at play, such as concerns about huge floodgates being an eyesore or political factors, but aims to do a straight cost-benefit analysis of which measures can control flooding at an acceptable cost. Aerts said he hoped to do a similar analysis for other cities, starting with locations in California.

The researchers analyzed three main types of flooding controls—large storm-surge barriers of various types holding back water from New York Bay, building improvements only and a "hybrid approach" combining infrastructure improvements with small levees and beach fortifications. All have been discussed in city circles, although the idea of huge flood barriers is not moving forward at this point, said Aerts.

The team simulated 549 different types of storms, ranging from small ones to hurricanes larger than Superstorm Sandy, and estimated the average resulting annual expected flood losses. The average damage per year of the simulated storms was $174 million, while the extreme simulated examples hit closer to $25.4 billion. The model's damage estimates for storms of Sandy's size matched what actually occurred, according to the researchers.

'Hybrid' solution with small barriers affordable now
Assuming a "middle climate change" scenario of about a foot of sea-level rise by midcentury, the team further assessed the cost-effectiveness of each flood-control strategy by measuring whether its benefits, or avoided risk, would outweigh the investment costs.

Under that test, only one of the huge barrier solutions made the cut as an affordable strategy. Called "NJ-NY connect," it envisions constructing two storm barriers—one in the East River and another connecting Sandy Hook in New Jersey with the tip of the Rockaways in Queens, New York.

Other huge floodwall concepts—like building three barriers to protect New York's Jamaica Bay—were too expensive in the short term before 2040, according to the study. Part of the reason is that they have much higher maintenance costs.

However, the most cost-effective idea of all was the "hybrid" approach, using a combination of infrastructure improvements, and smaller flood barriers to keep water out of the city. This hybrid approach—while not as effective as keeping out water as a large floodgate—would keep damage to a minimum while keeping costs lower. "Houses are safe, as well as subways and critical infrastructure" under the hybrid approach, said Aerts.

The city—and the federal government—largely are following this approach now, via a $20 billion climate resiliency plan announced in 2013 from outgoing Mayor Michael Bloomberg and other measures like beach restoration, he said. "The potential damage is going down," he said.

The affordability of the hybrid model means that it is a better option through about 2030, assuming sea-level rise follows the average projections of scientists, said Aerts. By 2040 or so, NJ-NY connect becomes an affordable option as well under this sea-level rise projection, he said.

Big flood barriers may be needed later
However, if climate change becomes worse than expected, and sea levels rise faster than projected, other huge flood barriers will become a viable financial option very quickly, he said. And midcentury and beyond, there likely will need to be large flood barriers combined with a more modest approach, regardless of what is cost-effective immediately, according to Aerts. In April, the American Geophysical Union released a study finding that the chance of storm-pushed water topping the Manhattan seawall is 20 times greater than 170 years ago, because of tide increases on top of sea-level rise.

The new analysis is not likely to end the debate about New York's options, and it also does not include all engineering ideas.

The Department of Housing and Urban Development, for example, has been overseeing a challenge, "Rebuild By Design," in which 10 teams vie for the best idea to promote resilience in the Sandy-affected region. The final proposals—released last month—range from a series of underwater "breakwater" walls around Staten Island to an offshore barrier island chain called "Blue Dunes."

There also is wide variation among projections of sea-level rise, as well as disagreement about investment costs of construction. Additionally, many experts have criticized the rebuilding efforts after Sandy as moving slowly and leaving the city vulnerable to similar or worse damage in the next decade (ClimateWire, Oct. 24, 2013).

Aerts encouraged more cooperation between New York and New Jersey, along with greater consideration—and in-depth studies—of large flood barriers, considering their decades-long implementation time. The early legwork needs to be going on now, he said.

The study also suggests boosting the flood control budget via charges to the pool of 50 million annual city tourists. "A simple $10 resilience fee—equivalent to the maximum September 11 airline security fee anyone traveling in the United States is now paying for a roundtrip ticket—could help," the study says.

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